1. Field of the Invention
The present invention relates to a phase shifter for adding deterministic phases to an original two dimensional information in order to make high quality and high storage density Fourier transformed holograms storing image information.
2. Description of the Prior Art
In recording a digital or an analog information on a photosensitive material by the holographic technique, it is common to use a Fourier transformed holography system, in which the hologram is recorded on the Fourier transformed plane in order to obtain high storage density. However, this Fourier transformed holography system has a disadvantage in that if the whole frequency components of the Fourier spectra of the original information should also be recorded precisely, saturation might occur in some bright portions of the Fourier spectra because of the non-linear characteristics of the photosensitive material used. Such saturation of the photosensitive material leads to a source of noise and results difficulties in obtaining a high diffraction efficiency of the hologram and thus a high quality reconstructed image is hardly realized.
In order to solve the abovementioned problem, the following two methods had been suggested:
(i) To shift the position of the photosensitive material at some distance from the exact Fourier transformed plane. This method is known as a defocus method. PA1 (ii) To provide a phase shifter having spatial phase distribution with respect to the original two dimensional information so as to distribute the Fourier spectra more evenly over the Fourier transformed plane.
If the method (i) is introduced to solve the problem, distance between the exact Fourier transformed plane and the photosensitive material should be made much longer. This results in disadvantages of lowering the information density and in the lack of redundancy of the hologram in addition thereto.
On the other hand, the second method (ii) of providing the phase shifter has a considerable effect. Generally the phase shifter consists of a number of unit cells arranged in a matrix shape, and the unit cells are given two level (0, .pi.) or four level (0, (1/2).pi., .pi., 3/2.pi.) phase shifts allotted at random. This random phase distribution is allotted by generating random numbers. Therefore, it is not decided deterministically so that a chosen distribution may not be guaranteed as a most suitable phase distribution.
Several methods of deterministic phase allocation for the unit cells forming such a phase shifter have been tried. A method of decision of such phase distribution in the case of providing a number of phase quantizations was reported already. However, if the number of the phase quantizations is increased, a disadvantage results in that the actual manufacture of the phase shifter becomes difficult.